Photoresists are used for transferring an image to a substrate. A layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to the activating radiation and other areas that are transparent to the activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist-coated substrate. Following exposure, the photoresist is baked and developed by contact with a developer solution to provide a relief image that permits selective processing of the substrate.
One approach to achieving nanometer (nm)-scale feature sizes in semiconductor devices is to use shorter wavelengths of light. However, the difficulty in finding materials that are transparent below 193 nm has led to the immersion lithography process to increase the numerical aperture of the lens by use of a liquid to focus more light into the film. Immersion lithography employs a relatively high refractive index fluid, typically water, between the last surface of an imaging device (e.g., KrF or ArF light source) and the first surface on the substrate, for example, a semiconductor wafer.
In immersion lithography, direct contact between the immersion fluid and photoresist layer can result in leaching of components of the photoresist into the immersion fluid. This leaching can cause contamination of the optical lens and bring about a change in the effective refractive index and transmission properties of the immersion fluid. In an effort to ameliorate this problem, use of a topcoat layer over the photoresist layer as a barrier between the immersion fluid and underlying photoresist layer has been proposed. The use of topcoat layers in immersion lithography, however, presents various challenges. Topcoat layers can affect, for example, process window, critical dimension (CD) variation and resist profile depending on characteristics such as topcoat refractive index, thickness, acidity, chemical interaction with the resist and soaking time. In addition, use of a topcoat layer can negatively impact device yield due, for example, to micro-bridging defects which prevent proper resist pattern formation.
To improve performance of topcoat materials, the use of self-segregating topcoat compositions to form a graded topcoat layer has been proposed, for example, in Self-Segregating Materials for Immersion Lithography, Daniel P. Sanders et al., Advances in Resist Materials and Processing Technology XXV, Proceedings of the SPIE, Vol. 6923, pp. 692309-1-692309-12 (2008). A self-segregated topcoat would theoretically allow for a tailored material having desired properties at both the immersion fluid and photoresist interfaces, for example, an improved water receding contact angle at the immersion fluid interface and good developer solubility at the photoresist interface.
Topcoats exhibiting a low receding contact angle for a given scan speed can result in water mark defects. These defects are generated when water droplets are left behind as the exposure head moves across the wafer. As a result, resist sensitivity becomes altered due to leaching of resist components into the water droplets, and water can permeate into the underlying resist. Topcoats having high receding contact angles would therefore be desired to allow for operation of immersion scanners at greater scan speeds, thereby allowing for increased process throughput. U.S. Patent App. Pub. Nos. 2007/0212646A1 to Gallagher et al. and 2010/0183976A1 to Wang et al. describe immersion topcoat compositions that include a self-segregating surface active polymer which allow for improved water receding contact angles. With the desire for increasingly faster scan speeds on the exposure tool to allow for increased throughput, topcoat compositions having further improved receding contact angles are desired.
There is a continuing need in the art for topcoat compositions exhibiting high receding contact angles for use in immersion lithography, and for photolithographic methods making use of such materials.